1. Field of the Invention
This invention relates to a method and apparatus for producing a hydrogen containing gas by steam reforming of methanol. This reformer is particularly suitable for use in an integrated fuel cell system for producing electric power from methanol.
2. The Prior Art
During recent years, industrial requirements for hydrogen have increased rapidly, and a variety of processes for the manufacture of hydrogen have been developed to fill this need. Large quantities of hydrogen are used, for example, in synthesis of ammonia; for catalytic hydrogenation, for example, of oils to solid fats; in petroleum processes such as hydrofining; and as a fuel, e.g. in missiles and in fuel cells for the generation of electricity.
Direct oxidation of fuels such as methanol in fuel cells at practical current densities with acceptable catalyst loadings is not as economically attractive as conversion of methanol fuel to a hydrogen-rich mixture of gases via steam reforming and subsequent electrochemical conversion of the hydrogen-rich fuel stream to direct current in the fuel cell.
A very attractive fuel cell system currently undergoing commercial consideration is the reformed methanol fuel-phosphoric acid electrolyte-air system. Primary advantages of phosphoric acid electrolyte (85 wt. %) include ability to operate with ambient air containing CO.sub.2, ability to operate with a thin matrix electrolyte (no liquid circulation) and chemical stability of the electrolyte over the operating temperature of the cell, e.g. 300.degree.-400.degree. F.
The fuel cell itself is only part of the overall system and other components of the system, e.g. generation of hydrogen is equally important in terms of overall system size and cost effectiveness.
In one method used by the prior art, hydrogen is produced by steam reforming methanol in a reactor which is shaped much like a conventional shell and tube heat exchanger except that the tubes contain catalysts. In these reactors, hot gases (typically combustion products) are passed through the shell of the heat exchanger while the methanol and water vapor is passed through the tubes. Thus, the heat required for the endothermic catalytic reforming reaction must pass through the wall of the tube. In these prior art processes, the mixture of methanol and steam is converted to a gaseous stream consisting primarily of hydrogen (about 68%) and CO.sub.2 (about 21.7%), CO (about 1.5%) and H.sub.2 O (about 8.8%). In order to improve the thermal and chemical efficiency of such reactors, efforts have been directed to improve the uniformity of heat distribution in the tubes within the reactor to secure high chemical conversion of fuel into hydrogen and maintain catalyst bed temperature within certain limits (&gt;700.degree. F.) in order to avoid premature catalyst aging while minimizing the amount of energy used to produce each unit of hydrogen containing gas.
For efficient operation of the steam reforming reaction, large surface areas are required to transfer the heat from the combusted gases to the tubes. In reformers presently used for steam reforming small diameter reaction tubes are clustered closely together in the furnace so that heat transfer from the combusting gases in the reactor into the catalyst packed tubes is optimized.
The use of a plurality of tubes to accomplish heat transfer contributes to the large size and high cost of the reformer. In fuel cell systems in which the reformer and the fuel cell are fully integrated, i.e. the combustion gases for the reforming reaction are derived from the fuel cell exhaust, the shell side heat transfer coefficient between the hot gas and the tube is characteristically low and hence, the rate of reaction is limited primarily by the rate of heat transfer. This problem is particularly severe at the reactor entrance as the rate of the endothermic reaction is very high, and thus, the amount of heat required is very high while the shell side heat transfer coefficient is often low as the mechanical design of typical reactors often allows the gases in the shell to be relatively stagnant near the tube entrances. This leads to a drop in the overall efficiency as a large portion of each reactor tube operates at an undesirably low temperature. Thus, in order to effect complete conversion, the reformer must be relatively large and expensive.